Many items in our everyday life are provided with coatings which have a protective, signal, or decorative function. In recent years, considerable effort has been expended to develop coating compositions with enhanced sustainability, that is coatings characterized by a low content of volatile organic compounds (VOC)—and which indeed may be solvent-free—and/or coatings which contain resins and additives that are based on renewable resources. In the latter regard, it is now well-known to employ unsaturated-fatty acid functionalized resins in coating compositions because such resins are largely derivable from agricultural products and are also easily biodegraded.
The oxidative air drying of compositions containing fatty acid functionalized resins—such an alkyd resins—is due to autoxidation and cross-linking of the unsaturated oil/fatty acid component of the resin with simultaneous evaporation of the carrier solvent(s). Absorption of oxygen from the air causes peroxide formation and peroxide decomposition, which results in the generation of free radicals (Bieleman, J. et al. “Chapter 7: Catalytically Active Additives” in Additives for Coatings, J. Bieleman (ed.) Wiley-VCH (2000)). The free radicals initiate cross-linking and formation of higher molecular weight polymers, eventually leading to a solidified “air dried” film or coating.
The time for such a composition to dry depends on the concentration and type of unsaturated oil used to prepare the resin. Autoxidation and crosslinking of the unsaturated oil/fatty acid component can proceed unaided, but the time for drying is generally found to be unacceptably long for many practical purposes. The reactions are significantly accelerated by the presence of a metal-based drying catalyst, commonly referred to as a “drier”. Whereas an alkyd coating may takes months to dry in the absence of a drying catalyst, in the presence of such a catalyst, drying can be accomplished within a few hours. The metal within the drying catalyst catalyzes autoxidation by forming a complex with both atmospheric oxygen and the double bonds of the unsaturated fatty acid groups within the composition.
Examples of known drier salts include polyvalent salts containing cobalt, calcium, copper, zinc, iron, zirconium, manganese, barium, zinc, strontium, lithium and potassium as the cation; and halides, nitrates, sulphates, carboxylates, such as acetates, ethylhexanoates, octanoates and naphthenates, or acetoacetonates as the anion.
The catalytic activity of the metal during decomposition of the (hydro)peroxide relies on the repeated transition of the metal ion from the lower to the higher oxidation state and back again, leading to reduction and oxidation of the hydroperoxides to catalyze and accelerate oxidation of the unsaturated oil component of the composition. For this reason, transition metals have more been commonly employed in such driers, as transition metals are capable of switching from a lower valence state to a higher valence state in a redox reaction with fatty acid peroxides present in the alkyd composition.
To date, driers based on cobalt have been most widely used because of their good performance at ambient temperature. However, because the cobalt salts will most likely be restricted in the near future because of regulatory issues, it is now desired to find alternative drier compounds that show at least comparable drying performance to that of cobalt driers and which can replace cobalt based driers completely in oxidatively air-drying coatings.
Driers based on non-cobalt metal salts, and in particular on manganese (Mn), are known from inter alia: EP 1 382 648 A1 (Van Gorkum et al); WO 2003/093384 (Oostveen et al.); E. Bouwman, R. van Gorkum, J. Coat. Technol. Res., 4, 491-503 (2007); and, R. van Gorkum et al., Journal of Catalysis 252 110-118 (2007). It is however considered that these prior art Mn-based driers: may not promote sufficient drying in a coating composition comprising an alkyd resin, especially in relation to tack free time; and, can yield coatings which suffers from severe dark yellowing.
Dinuclear manganese based complexes, [MnIV2(μ-O)3L2](PF6)2 (or MnMeTACN) wherein L is 1,4,7-trimethyl-1,4,7-triazacylononane have been disclosed as catalysts for the oxidative drying of alkyd paints (Oyman et al., Surface Coatings International Part B: Coatings Transactions, Vo. 88, B4, 231-315, December 2005). WO2011/098583, WO2011/098584 and WO2011/098587 (all DSM IP Assets B.V.) suggested that the alkyd coatings of Oyman did not dry with the desired efficiency and were prone to deleterious skin formation when stored inside a pot. Accordingly, these three citations have proposed modifications to the MnMeTACN catalyst wherein: the bridging oxygen (μ-O) is optionally replaced by organic residues; and/or one or more methyl of 1,4,7-trimethyl-1,4,7-triacylcyclononane is optionally replaced by substituted or un-substituted C2-C20 alkyl groups or by substituted or un-substituted C6-C20 aryl groups; and/or where the (PF6)2 anion is optionally replaced by a carboxylate anion.
The crystal structure of several binuclear manganese complexes have been resolved, see for example Wieghart et al., J.A.C.S. 110(22):7398-7411 (1988) and Romakh et al., Inorg. Chim. Acta 359(5):1619-1626 (2006).
Despite these developments, there still remains a strong need in the art for alternative or better non-cobalt catalysts which can provide for reduced yellowing of the coatings compositions which contain them, which concomitantly provide for fast drying coatings which are characterized by beneficial hardness and gloss properties. Preferably the coating composition has good storage stability without the need to provide for adjunct ingredients in larger amounts than commonly used, such as anti-skinning compounds, in the coating compositions.